The beating of the chest is relatively new since it didn’t happened before the liturgical changes a few years ago.
(However, it is a sign or remorse which stretches back for millenia.)
To be a mortal sin, one must 1. commit a grave (serious) wrong; 2. one must understand that it is gravely wrong; and 3. one must fully intend to commit the gravely wrong aspect of the act.
Avoiding beating one’s chest at Mass is not seriously wrong; therefore it is not a mortal sin right off the bat.